Hitherto, it has been customary with oil pumps for internal combustion engines that in pumps of the type having direct connection to the engine, internal gearing of involute tooth including an outer rotor 3 having teeth 1 and an inner rotor 4 having teeth 2 profile as shown in FIG. 1 is employed and that where internal gearing of trochoid tooth profile is employed operation is carried out by reducing the number of revolutions through an idle gear or a belt pulley. Recently, fuel cost saving in internal combustion engines has been widely recognized as a vital need, and the same has now been called for with respect to oil pumps as well.
In the internal gear pump having an involute tooth profile, a crescent 5 which is structurally necessary as such is a major cause of lowering mechanical efficiency, with volumetric efficiency .eta..sub.v =60.about.80%, mechanical efficiency .eta..sub.m =15.about.30%, and total efficiency .eta.=10.about.25% being typical under a normal operating pressure of 5.about.6 Kg/cm.sup.2. In, in the internal gear pump having a trochoid tooth profile, if the crescent is eliminated, the following efficiencies are obtainable under a comparable operating pressure: volumetric efficiency .eta..sub.v =90.about.100%, mechanical efficiency .eta..sub.m =30.about.60%, and total efficiency .eta.=30.about.60%.
However, the trouble with a conventional internal gear pump having a trochoid tooth profile is that if it is operated at a high rotational speed in the same manner as one having direct connection to the engine, cavitation is likely to take place, or eccentric wear due to high speed is likely to develop.